Well depending on how much you are going to be feeding them, i'd suggest a weekly water change. I've known some people ( my friends) who really feed their bettas a few times a day, and the tank is kinda dirty, and they end up having to clean their tank 2x a week about a 50% water change. It just kinda depends on how much you feed them. Bettas arnt usually that dirty, but you would be suprised to see how much stuff you see when you siphon it ( if thats how you plan on cleaning it)

I found the best stuff to cycle my tank with...trust me i have 2 saltwater fish and they are really tough to keep alive in a tank that is not already cycled and mine have been doing great for a month now. its called stress zyme by API. It works soooo good! Not sure whether you can use it for bettas or not, but it works for both fresh and saltwater fish. I would imagine you can, says its safe for all fish, and its not expensive either.

I have heard that overcrowding thing too. It was written by a guy who says he's never had a fish kill another, and they are mostly healthy. I wasn't sure about whether it was true or not because it went against everything I had heard. But I did hear that too. lol